Base thickness and high-frequency performance of SiGe HBT´s

Summary form only given. The influence of the base thickness on the high-frequency performance of about 20 fabricated HBTs (heterojunction bipolar transistors) has been analyzed by plotting the highest obtained f_T value against base thickness. Base thickness is defined as the distance between the two Si/SiGe interfaces which usually coincide with the metallurgical pn-junctions. A steady increase of f_T with decreasing base thickness can be clearly seen: starting from the first published HBTs with 50 nm base thickness which did not exceed 20 GHz, transistors with 40 nm base had 37-52 GHz and devices with 28-30 nm bases exhibited 58-91 GHz. The latest two samples had only 25 and 22 nm base thicknesses, leading to an f_T of 95 GHz (f_max=50 GHz), the highest value reported for Si transistors. The base doping in the latter sample was 8×10¹⁹ cm^-3 to maintain a base sheet resistance of about 1.2 kΩ